To be able to significantly reduce a pollutant emission, in particular of motor vehicles, today, the so-called SCR method is used which contributes to reduce the nitrogen oxide emission of diesel vehicles. The ammonia required for the SCR reaction is not used directly, i.e. in pure form, but usually in the form of an aqueous urea solution. From this urea solution, ammonia and carbon dioxide are generated through a hydrolysis reaction, wherein the ammonia generated in this manner can react in a special SCR catalyst at an adequate temperature with the nitrogen oxides of the exhaust gas. The consumption of urea solution depends on the raw emission and in particular also on a load condition of the motor vehicle so that always a certain volume of urea solution must be carried along in a tank provided for this purpose. Since the urea solution contains water for dilution, there can be the danger at extreme temperatures that the aqueous urea solution freezes in the tank provided for this purpose and, therefore, the SCR reaction required for the nitrogen oxide reduction can not take place. Thus, it is essential for a reliable operation and for compliance with the statutory emission standards that the freezing of the urea solution can be reliably prevented. Since in the known urea tanks usually also filter devices for filtering the aqueous urea solution are provided which can have a tendency to clog at low temperatures, moreover, it must be ensured that also such filter devices work without problems and properly at all occurring temperatures.